Diffractometry relates to the irradiation of a material or object using a source of electromagnetic radiation, of X or gamma type, followed by the analysis of radiation from elastic scattering at a small angle. The expression “radiation from elastic scattering at a small angle” designates the radiation coherently scattered by the material or object at an angle less than 15°, or less even than 10°, relative to the direction of the radiation that is incident on the material or object. As a matter of fact, beyond 10°, elastic scattering, or Rayleigh scattering, becomes progressively negligible. It is known to use diffractometry to detect certain crystalline substances such as most explosives or numerous other dangerous or illegal structures.
The invention thus finds an application in the field of security, more particularly in the field of the detection of explosive materials in an item of baggage.
It is also useful in the medical field, for example for locating a tumor in a breast. More particularly, a publication by UCL (Pani, S. et al. “Characterization of breast tissue using energy-dispersive X-ray diffraction computed tomography”. Applied Radiation and Isotopes 68, No. 10 (2010): 1980-1987) has been able to show the possibility of differentiating (adipose tissues, fibrous tissues, benign tumors, fibroadenomas, carcinomas, etc.) objects of biological tissues from breast biopsies thanks to the scattering measurement of these tissues.
The analysis of the radiation scattered at a small angle (it is to be noted that that the term “diffracted” is generally used for a crystalline material, whereas the term “scattered” is generally used for an amorphous material, but these two terms here are used interchangeably, and likewise for the terms scattering and diffraction) by a material is a method of physico-chemical analysis which provides information on the structure of the material thereby enabling better characterization of materials.
It is known that the analysis of the spectrum of the radiation scattered at a small angle, or scattering spectrum, makes it possible to establish a signature for the material examined.
For crystalline materials for example, when the wavelength of the irradiating X-rays is of the same order of magnitude as the interplanar spacing (a few angstroms), the scattered rays generate constructive or destructive interferences according to their energy and their scattering angle. The conditions for which the interferences are constructive are determined by Bragg's law. For a crystalline material, this law links the interplanar spacing, the scattered radiation energy and the scattering angle, according to the following equation:
      E    hkl    =      n    ⁢          hc              2        ⁢                  d          hkl                ⁢                  sin          ⁡                      (                          θ              /              2                        )                              with:dhkl: interplanar spacing between the crystallographic planes of the irradiated crystal;{hkl}: Miller indicesθ: scattering angle, that is to say the angle formed between the scattered radiation analyzed and the beam that is incident on the irradiated crystal;h: Planck's constant;c: the speed of light;n: the order of the interference.
It is possible to identify the Bragg peaks by the momentum transfer defined by the following equation:
  x  =                    sin        ⁡                  (                      θ            /            2                    )                    λ        =          n              2        ⁢                  d          hkl                    
The interest in expressing the scattering profiles (measured intensity) according to x is due to the fact that an intensity peak may be measured for different pairs (λ, θ) but for a single value of x (n fixed). By way of examples, appended FIG. 1 shows the Bragg peaks of two crystals, TNT (trinitrotoluene) and salt (NaCl).
In the case of non-crystalline materials, the spectrum for scattering at a small angle is also representative of the material examined.
In the manner of the interferences determined by Bragg's law for a crystalline material, interference phenomena may also occur between the atoms and/or molecules of an amorphous material such as a liquid, this time involving a known distribution of distances (molecular interference function, denoted MIF). As a matter of fact, many amorphous materials have regular arrangements over nanometric distances (the expression short-range order used). This type of order is determined by strong chemical bonds for the covalent and ionic bonds. This short-range order causes intramolecular and intermolecular interferences. Appended FIG. 2 illustrates examples of molecular interference functions, i.e. the normalized MIF for water (H2O), the normalized MIF for oxygenated water (H2O2), the normalized MIF for acetone (C2H6CO), and the normalized MIF of a material known under the tradename Plexiglas® ((C5O2H8)n).
The most common diffractometers are referred to as ADXRD (acronym for “Angular Dispersive X-ray Diffraction”). The energy is fixed by the use of monochrome radiation and the number of photons diffracted is measured according to the angle. Although these devices are very accurate, they require the use of a powerful monochrome source and cannot be used for imaging on account of their bulk.
Developed more recently, the EDXRD technique (EDXRD being the acronym for “Energy Dispersive X-Ray Diffraction”) enables these difficulties to be alleviated. This time they consist in working at a fixed angle and using a set of collimators to illuminate the object with a polychromatic beam to measure the diffracted photons with an energy resolved spectrometric detector. The diffraction peaks then appear at certain energies in the measured spectrum.
The EDXRD technique, and more generally any technique of analysis by spectrometry, requires the employment of a spectrometric detector that is sufficiently energy resolving to enable the separation and the identification of the different characteristic peaks of the material constituting the object to analyze. The known detectors having the best energy resolution are of the Germanium type. However, this type of detector must be cooled to very low temperatures, by complex and/or costly methods (thermoelectric cooling or cooling by a tank of liquid nitrogen). Also, the analysis devices employing such a detector are very bulky.
The recent emergence of spectrometric detectors capable of being used at ambient temperature, such as detector types implementing CdTe, CdZnTe, or scintillator materials, provides an attractive alternative to the Germanium detectors. To be precise, these detectors are compact, not cooled and less costly. However, their performance in terms of energy resolution is still less than that obtained with the Germanium detectors, even though quite good.
To know whether a given crystalline or amorphous substance is contained in an object, it is thus known to:                irradiate the object using an incident beam, emitted by a source of ionizing radiation, preferably collimated by a primary collimator,        detect the diffracted radiation using a detection device comprising                    a detector, here termed spectrometric detector, configured to establish an energy spectrum of the radiation scattered at a given scattering angle, that is to say a detector comprising                        a detector material capable of interacting with radiation scattered by the object and which, on the side facing the object, presents what is referred to as a detection plane,        spectrometry measurement means, configured to measure an energy released by each interaction of a photon with the detector material and to establish at least one energy spectrum.                    a collimator, referred to as detection collimator, associated with the detector, the detector and the detection collimator being arranged so as to have a detection axis D forming a scattering angle θ with the central axis Z of the incident beam,                        analyzing the measured spectrum or spectra by comparison with the energy spectrum of the substance searched for.        
In general terms, an energy spectrum illustrates the energy distribution of radiation in the form of a histogram representing the number of photon interactions in the object (along the y-axis) according to the released energy (along the x-axis). Often, the energy axis is discretized into channels of width 2 δE, a channel Ci, centered on the energy Ei corresponding to the energies comprised between Ei−δE and Ei+δE.
It is known that the identification of a material by the production then analysis of a scattering spectrum may be painstaking when it is wished to characterize objects of high volume. More particularly, this method is all the more accurate the smaller the volume of the object observed.
Furthermore, from WO 2006/075296 there is known a device and a method for identifying materials comprising a polychromatic source of X-rays, a detector configured for detecting radiation that is scattered (or diffracted) and radiation that is transmitted by the analyzed object, and a computing unit for:                reconstructing an image of the object by tomography from the transmitted radiation, and comparing that transmission image with reference images to determine whether there is a suspicious region in the analyzed object,        determining a first differential effective coherent scattering cross-section corresponding to a first momentum transfer of the detected scattered radiation for a suspicious region, and comparing this first differential effective cross-section with a first value corresponding to materials searched for,        determining a second differential effective coherent scattering cross-section corresponding to a second momentum transfer of the detected scattered radiation for the suspicious region, and comparing this second differential effective cross-section with a second value corresponding to the materials searched for,        determining a total effective cross-section for the suspicious region by summing the first and second differential effective cross-sections, and comparing that total effective cross-section with a third value corresponding to the materials searched for.        
The length of execution of the method of WO 2006/075296 makes it incompatible with certain applications such as the search for explosive or dangerous products in baggage.